1. Field of the Invention
The present invention relates to an imaging lens system, an imaging optical device, and a digital appliance. More particularly, the present invention relates to, for example, a compact, large-diameter imaging lens system suitable for an interchangeable-lens digital camera that takes in an image of a subject with an image sensor (e.g., a solid-state image sensor such as a CCD (charge-coupled device) image sensor or a CMOS (complementary metal-oxide semiconductor) image sensor), an imaging optical device that outputs the image of the subject thus taken in with the imaging lens system and the image sensor in the form of an electrical signal, and a digital appliance equipped with an image input function, such as a digital camera, that incorporates the imaging optical device.
2. Description of Related Art
Conventionally, imaging lens systems with fast f-numbers (i.e., with smaller f-numbers) have been sought. On the other hand, in recent years, digital single-lens-reflex cameras have been adapted to increasingly high resolutions, and in addition it has been becoming increasingly easy for users to check images on a true-to-pixel-size basis. Accordingly, increasingly high optical performance is required in imaging lens systems, and high uniformity of image quality across the entire image is sought eagerly even at the shortest shooting distance. Moreover, in mirrorless interchangeable-lens digital cameras, which have no swing-up mirror any more, the focusing group is required to be lightweight to permit faster focusing by contrast AF (autofocus). To meet these needs, imaging lens systems of a type that is suitable to reduce the weight of the focusing group while maintaining high optical performance are proposed in Patent Documents 1 and 2 identified below.
Patent Document 1: Japanese Patent Application published as No. 2014-48488
Patent Document 2: Japanese Patent Application published as No. 2014-238608
However, in the imaging lens system disclosed in Patent Document 1, the optical power of the first lens group is so low relative to the optical power of the entire system that no sufficient optical power is gained in the first lens group; thus, a large part of the optical power of the entire system has to be accounted for by the third lens group. As a result, spherical aberration cannot be corrected satisfactorily, and this makes it impossible to achieve a diameter large enough to provide an f-number of about 1.4 combined with satisfactory aberration correction.
On the other hand, in the imaging lens system disclosed in Patent Document 2, fewer lens elements are used in the focusing group to reduce its weight and thereby achieve faster focusing. However, the optical power of the first lens group is so low relative to the optical power of the second lens group, which is the focusing group, that most of the overall optical power is accounted for by the second lens group. Generally, as the optical power of the focusing group increases, the variation of aberrations that accompanies variation of the shooting distance increases, and this makes it difficult to correct aberrations satisfactorily from the infinity-shooting condition to the close-distance-shooting condition.